Back in February, all the major search engines (Google, Yahoo, MSN and Ask) agreed to recognize the syntax of the "canonical tag". The canonical tag is essentially a new "link element" for the HTML on your website pages that is intended to clean up duplicate URLs on sites.
So, for example, sometimes you have http://www.example.com as your URL and sometimes it's http://www.example.com/?sessionID649AY56 as the URL of the same page. The canonical tag specifies which is the "real" URL to help search engines avoid indexing duplicate content.
Explaining the use and implementation of the canonical tag in further detail is this great overview by Helen Overland.
I also found this video Q&A from Matt Cutts (Google Guru) on how large corporations with large websites should handle the canonical tag and whether it should be on every page.
There is also this great piece from Matt which includes links to help centre resources and other posts about the canonical tag.Follow us!